Process oil, high-viscosity base oil, and process for the production thereof

ABSTRACT

A novel process for the production of an extract useful as a process oil and a raffinate useful as a high-viscosity base oil by solvent refining is provided, characterized in that reduced pressure distillation is effected under the condition that the end point of distillate is 580° C. or higher as calculated in terms of atmospheric pressure or the initial boiling point of the residue is 450° C. or higher as calculated in terms of atmospheric pressure, the resulting residual oil is deasphalted under the condition that the carbon residue content in the deasphalted oil reached 1.6% or less, and the resulting deasphalted oil is subjected to solvent refining under the condition that the yield of extract is from 35% to 60%. It is a novel and economically excellent process for the preparation of a rubber process oil having a high safety, a high penetrating power with respect to rubber polymer and the content of PCA extract of less than 3%.

FIELD OF THE INVENTION

[0001] The present invention relates to a process oil for the additioninto natural rubber and synthetic rubber and a high-viscosity base oil,as well as to a process for the production thereof. More particularly,the present invention relates to a rubber process oil which has a lowcontent of a polycyclic aromatic compound so as to exhibit no toxicityor carcinogenicity and can be easily handled and to a process for theproduction thereof.

BACKGROUND OF THE INVENTION

[0002] A rubber process oil is used to facilitate the procedure such askneading, extrusion and molding in the production of rubber byexhibiting a penetrating power with respect to rubber polymer structure.A rubber process oil is also used to improve the physical properties ofrubber products. Such a rubber process oil is required to have anappropriate affinity for rubber. On the other hand, examples of rubbersto be processed include natural rubber and synthetic rubber. There arevarious synthetic rubbers. Among these rubbers, natural rubber andstyrene-butadiene rubber (SBR) are often used. Therefore, a rubberprocess oil having a large amount of aromatic hydrocarbon and a highaffinity for rubber is normally used.

[0003] The rubber process oil is obtained by extracting a lubricantfraction obtained by distillation of crude oil under reduced pressure oran oil obtained by deasphalting reduced pressure distillation residuewith a solvent having an affinity for aromatic hydrocarbon. The rubberprocess oil thus obtained contains an aromatic compound in an amount offrom 70% to 99% as determined by column chromatography, exhibits apercent C_(A) of from 20% to 50% according to ring analysis (ASTM D2140)and contains the content of PCA (polycyclic aromatic compound) extractof from 5 to 25% by mass. The content of PCA extract is defined by IP346method of British Society of Petroleum.

[0004] However, the carcinogenicity of PCA has recently been noticed. InEurope, the law stipulates that oils having the content of PCA extractof 3% or more shall have an indication of toxicity. There is a movementto regulate the use of these oils. Accordingly, it is of urgentnecessity to reduce the content of PCA extract of rubber process oil toless than 3%.

[0005] Referring to rubber process oil having the content of PCA extractof less than 3%, JP-W-6-505524 discloses a process for the production ofa rubber process oil having the content of PCA extract of less than 3%which comprises deasphalting the residue of distillation under reducedpressure, and then dewaxing the oil thus obtained (the term “JP-W” meansa published Japanese translation of a PCT application).

[0006] The foregoing oil has the low content of PCA extract but has ahigh aniline point. The aniline point is an index of the content ofaromatic hydrocarbon. A high aniline point means a low aromatichydrocarbon content. However, when the content of aromatic hydrocarbonin an oil is decreased, the resulting oil exhibits a lowered affinityfor rubber. Therefore, the rubber process oil disclosed in the abovecited patent publication exhibits deterioration of properties requiredfor rubber process oil, i.e., penetrating power with respect to rubberpolymer. Further, it is made difficult to provide the final rubberproduct with satisfactory physical conditions.

[0007] JP-W-7-501346 discloses a noncarcinogenic bright stock extractand/or deasphalted oil and a process for the production thereof, andproposes to use characteristics related to mutagenicity index (MI) as anindex of purification to reduce MI to 1 or less. In this case, an oilobtained by deasphalting the residue in a vacuum distillation column, anoil having a reduced aromatic compound content obtained by extracting adeasphalted oil or an oil obtained by dewaxing the foregoing oil isused. However, it is considered that the content of PCA extract is 3% ormore. The relationship between MI and the content of PCA extract of sucha deasphalted oil is not disclosed in the above cited patentpublication.

[0008] The present invention is to solve the foregoing problems. It istherefore an object of the present invention to provide a rubber processoil having a high safety, a high penetrating power with respect torubber polymer and the content of PCA extract of less than 3% and anovel and economically excellent process for the preparation thereof.

SUMMARY OF THE INVENTION

[0009] As a result of extensive studies to achieve the foregoing objectof the invention, the present inventors found that the content of PCAextract reaches less than 3% under specific distillation and solventrefining conditions. The present invention has thus been accomplished.

[0010] Based on the above finding, the present invention provides:

[0011] 1. A process for the production of an extract useful as a processoil and a raffinate useful as a high-viscosity base oil by solventrefining, which comprises

[0012] carrying out a reduced pressure distillation under the conditionthat the end point of distillate as converted to the value underatmospheric pressure is 580° C. or higher or the initial boiling pointof the residue is 450° C. or higher as calculated in terms ofatmospheric pressure,

[0013] deasphalating the resulting residual oil under the condition thatthe carbon residue content in the deasphalted oil reaches 1.6% or less,and

[0014] subjecting the resulting deasphalted oil to solvent refiningunder the condition that the yield of extract is from 35% to 60%.

[0015] 2. The production process according to 1 above, wherein theextract useful as a process oil exhibits a 100° C. dynamic viscosity offrom 50 to 100 mm²/s, a percent C_(A) of from 15% to 35%, the content ofPCA extract (IP346) of less than 3%, an aniline point of 90° C. orlower, a chromatographically-determined aromatic content of from 60% to95% by weight and Mw (weight-average molecular weight) of 650 or more.

[0016] 3. The production process according to 1 above, wherein thehigh-viscosity base oil having a 40° C. dynamic viscosity of from notlower than 400 mm²/s to not higher than 700 mm²/s obtained after thedewaxing of raffinate exhibits a pour point of not higher than −5° C.and a viscosity index of not lower than 95.

[0017]4. A process oil having a 100° C. dynamic viscosity of from 50 to100 mm²/s, a percent C_(A) of from 15% to 35%, the content of PCAextract (IP346) of less than 3%, an aniline point of 90° C. or lower, achromatographically-determined aromatic content of from 60% to 95% byweight and Mw (weight-average molecular weight) of 650 or more.

[0018] 5. The process oil according to 4 above, which has a mutagenicityindex MI of less than 1.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be further described hereinafter.

[0020] In general, a process oil can be prepared from petroleum,particularly from a lubricant fraction derived from crude oil, as astarting material. The lubricant fraction can be obtained as a fractionwhen the residue obtained by atmospheric distillation of crude oil isdistilled under reduced pressure or as a deasphalted oil when theresidue obtained by reduced pressure distillation of atmospheric residueis deasphalted. As a method for separating the constituents of thelubricant fraction from each other there is used solvent refining. Whenthe lubricant fraction is subjected to solvent refining with a solventhaving a selective affinity for an aromatic hydrocarbon compound, thearomatic hydrocarbon can be separated from the lubricant fraction. Theextract thus obtained contains a large amount of high boiling pointaromatic compounds.

[0021] Since a solvent having a selective affinity for an aromatichydrocarbon has a higher affinity for PCA than for an aromatichydrocarbon, the aromatic hydrocarbon extracted by ordinary extractionmethod contains a large amount of PCA. If PCA can be removed from theextract, a suitable process oil can be obtained.

[0022] The inventors conducted studies of process for the production ofan oil having a reduced content of PCA. As a result, it was found thatan oil having a reduced content of PCA can be effectively produced bycombining specific distillation and solvent refining conditions.

[0023] In accordance with embodiments of the process for the productionof the rubber process oil according to the present invention, alubricant fraction obtained by reduced pressure distillation of crudeoil or a deasphalted oil fraction obtained by deasphalting theatmospheric or reduced pressure distillation residue of crude oil istreated with a solvent having an affinity for aromatic hydrocarbon. Thesolvent and the resulting extract are then separated and recovered. Theraffinate separated during the solvent extraction may be subjected tohydrogenation/dewaxing, if necessary, and used as a high-viscosity baseoil.

[0024] The process oil obtained by each of these embodiments of theproduction of the rubber process oil according to the present inventionis the most suitable rubber process oil having a lower content ofpolycyclic aromatic compound but rich with aromatic hydrocarbon.

[0025] The conventional definition of PCA may include an aromaticcompound having three or more cycles, but the IP346 method is anordinary and standard method approved as a method for determining PCAcontent in oil material.

[0026] Since the rubber process oil obtained according to the productionprocess of the present invention has an extremely low content ofpolycyclic aromatic compounds but shows little or no decrease in thechromatographically-determined aromatic hydrocarbon content as comparedwith the conventional rubber process oil. Thus, the rubber process oilof the present invention has a high penetrating power with respect torubber such as SBR rubber and natural rubber and thus does not lower theworkability of rubber. In addition, the rubber process oil of thepresent invention is a material which can provide a rubber exhibitingphysical properties of the almost same level as that of rubber productsobtained by treatment with a conventional process oil containing muchPCA.

[0027] Embodiments of the production process of the present inventionwill be further described hereinafter.

[0028] In order to produce a rubber process oil of the presentinvention, crude oil is subjected to atmospheric distillation. Theatmospheric residue is then subjected to reduced pressure distillation.The resulting residue is then deasphalted. The deasphalted oil fractionthus obtained is then treated with a solvent having a selective affinityfor aromatic hydrocarbon to remove raffinate therefrom. In this manner,an extract is obtained in the form of mixture with the solvent. A rubberprocess oil can be obtained by removing the solvent from the mixture.

[0029] Deasphalted oils obtained by deasphalting the residue ofdistillation under reduced pressure of the atmospheric residue ofvarious crude oils such as paraffin oil and naphthalene oil can bepreferably used.

[0030] The reduced pressure distillation may be carried out under thecondition that the end point of distillate is 580° C. or higher ascalculated in terms of atmospheric pressure or the initial boiling pointof the residue is 450° C. or higher as calculated in terms ofatmospheric pressure.

[0031] The end point of distillate which is lower than 580° C. is notpreferable, because the resulting extract would have the high content ofPCA extract.

[0032] Subsequently, the residue obtained by reduced pressuredistillation is deasphalted under the condition that the carbon residuecontent in the deasphalted oil reached 1.6% or less. The carbon residuecontent in the deasphalted oil exceeding 1.6% is not preferable, becausethe resulting extract would have an increased PCA content and theoxidation stability of the high-viscosity base oil obtained as araffinate would be adversely influenced.

[0033] The deasphalted oil thus obtained is then subjected to solventrefining, i.e., extraction with a solvent having an affinity foraromatic hydrocarbon. Examples of the solvent having a selectiveaffinity for aromatic hydrocarbon include furfural, phenol orN-methyl-2-pyrrolidone, singly or in combination of selected two or morethereof.

[0034] The solvent refining is effected under the condition that theyield of extract becomes from 35% to 60%. The solvent refining under thecondition that the yield of extract falls below 35% is not preferable,because the content of PCA extract would not fall below 3%. On thecontrary, the solvent refining under the condition that the yield ofextract exceeding 60% is not preferable, because the resulting extractwould exhibit a reduced aromatic content and the yield of thehigh-viscosity base oil obtained as a raffinate would be reduced tolower the economy.

[0035] Specific extraction conditions under which the yield of extractfalls within the above defined range depend on the composition of thedeasphalted oil to be processed and thus cannot be unequivocallydetermined. In practice, however, the extraction conditions can beadjusted by the solvent ratio, pressure, temperature, etc.

[0036] In general, the deasphalted oil is brought into contact with thesolvent at a temperature of generally 60° C. or higher, preferably from60° C. to 155° C., and a solvent/oil ratio of about 2/1 to 7/1 (byvolume) to remove the raffinate -therefrom. The raffinate thus removedmay be subjected to hydrogenation/dewaxing as necessary so that it isused as a high-viscosity lubricating base oil.

[0037] The extract useful as a process oil in the present inventionexhibits a 100° C. dynamic viscosity of from 50 to 100 mm²/s, a percentC_(A) (ASTM D2140) of from 15% to 35%, the content of PCA extract(IP346) of less than 3%, an aniline point of 90° C. or lower, achromatographically-determined aromatic content of from 60% to 95% byweight and Mw (weight-average molecular weight) of 650 or more. Theextract also exhibits a mutagenicity index MI of less than 1.

[0038] The 100° C. dynamic viscosity of the extract exceeding 100 mm²/sis not preferable, because the extract exhibits a lowered workabilitywhen used as a process oil and the extract does not exert a sufficienteffect of lowering viscosity with respect to rubber when used as aprocess oil. On the contrary, the 100° C. dynamic viscosity of theextract falling below 50 mm²/s is not preferable, because it becomesextremely difficult to reduce the content of PCA extract to less than 3%and the economical efficiency of refining process is lowered.

[0039] When the percent C_(A) (ASTM D2140) of the extract falls below15%, it would be difficult to produce a rubber using a rubber processoil and there is a possibility that the resulting rubber products havedeteriorated physical properties. On the contrary, when the percentC_(A) (ASTM D2140) exceeds 35%, there is a possibility that theresulting rubber products have deteriorated physical propertiessimilarly to the case where the percent C_(A) (ASTM D2140) falls below15% and it might be extremely difficult to reduce the content of PCAextract to less than 3% and the economical efficiency of refiningprocess is lowered.

[0040] The content of PCA extract (IP346) should be below 3% because thecontent of PCA extract of 3% or more conflicts with EU regulations forthe reason that it can be carcinogenic.

[0041] The aniline point of the extract exceeding 90° C. is notpreferable, because the affinity to a rubber is lowered.

[0042] When the chromatographically-determined aromatic content of theextract falls below 60%, there is a possibility that the production of arubber using a rubber process oil becomes difficult and that theresulting rubber products have deteriorated physical properties. On thecontrary, when the chromatographically-determined aromatic content ofthe extract exceeds 95%, there is a possibility that the resultingrubber products have deteriorated physical properties and it would bedifficult to reduce the content of PCA extract to less than 3% and theeconomical efficiency of refining process is lowered.

[0043] The Mw (weight-average molecular weight) of the extract fallingbelow 650 is not preferable, because it would be extremely difficult toreduce the content of PCA extract to less than 3% and the economicalefficiency of refining process is lowered.

[0044] When the mutagenicity index MI of the extract is 1.0 or more, theresulting product can be carcinogenic and thus it is not preferable.

[0045] The glass transition point of the extract determined by adifferential scanning calorimeter (DSC) is preferably not lower than−70° C. because the resulting extract exerts an improved effect ofproviding the rubber products with reduced loss. From the standpoint oflow temperature properties, the glass transition point of the extract ispreferably not higher than −20° C.

[0046] The raffinate obtained by solvent refining is optionallyperformed to hydrogenation/dewaxing to obtain a high-viscosity base oilhaving a pour point of not higher than −5° C., a viscosity index of notlower than 95 and a dynamic viscosity (40° C.) of from 400 mm²/s to 700mm²/s.

[0047] In accordance with the production process of the invention, theextract obtained by the one-step solvent extraction can be used as aproduct as it is, making it possible to reduce the production cost ascompared with the two-step solvent extraction process or the processrequired second step such as hydrogenation.

[0048] The production process of the invention makes it possible toobtain a noncarcinogenic process oil and a high-viscosity lubricatingbase oil, VI of which is higher than usual at the same time, giving anexcellent economy.

[0049] The present invention will be further described-in the followingexamples, but the present invention should not be construed as beinglimited thereto.

[0050] The various properties of the invention were determined accordingto the following methods.

[0051] Measurement of concentration of polycyclic aromatic compound(PCA):

[0052] The content of PCA extract was determined by IP346 testing method(edition of 1992).

[0053] Ring analysis:

[0054] The ring analysis percent C_(A) was calculated according to ASTMD 2140-97.

[0055] The dynamic viscosity was measured according to the methoddefined in JIS K2283-1993.

[0056] Aniline point:

[0057] The aniline point was measured according to the method defined inJIS K2256-1998.

[0058] Mw (weight-average molecular weight):

[0059] Mw is defined as ΣMi²Ni/ΣMiNi (Mi: molecular weight; Ni: numberof mols). Mw is generally measured by GPC (gel permeationchromatography).

[0060] Mw was measured by GPC under the following conditions (inpolystyrene equivalence).

[0061] Solvent: Tetrahydrofuran

[0062] Column temperature: 50° C.

[0063] Flow rate: 1.0 ml/min.

[0064] Column: Shodex GPC KF-805L

[0065] Detector: Shimadzu RID-6A

[0066] Pour point:

[0067] The pour point was measured according to the method defined inJIS C2101-1999.

[0068] Viscosity index:

[0069] The viscosity index was calculated according to the methoddefined in JIS K2283-1993.

[0070] Nitrogen content:

[0071] The nitrogen content was calculated according to the methoddefined in JIS K2609-1998.

[0072] Sulfur content:

[0073] The sulfur content was measured according to the method definedin JIS K2541-1996.

[0074] Chromatographically-determined aromatic content:

[0075] The chromatographically-determined aromatic content was measuredaccording to the method defined in ASTM D2007-98.

[0076] Mutagenicity index (MI):

[0077] The mutagenicity index (MI) was measured according to the methoddefined in ASTM E1687-98.

[0078] The gas chromatographic distillation was measured according tothe method fined in ASTM 2887-97a.

[0079] The carbon residue content was measured according to the methoddefined in JIS K2270-1998.

EXAMPLE 1

[0080] The atmospheric residue of Arabian light crude oil was distilledunder reduced pressure until the end point (gas chromatographicdistillation FBP) reached 600° C. The resulting residue was thendeasphalted with propane (solvent ratio: 700%; pressure: 3.3 MPaG;reaction column temperature: 72° C.) so that the carbon residue contentreached 1.3%. The deasphalted oil was then subjected to solventextraction with furfural as a solvent at a solvent ratio of 400% so thatthe yield of extract reached 42%.

[0081] The raffinate thus obtained was subjected to purification byhydrogenation in the presence of an alumina-based catalyst having 3 wt %of nickel and 12 wt % of molybdenum supported thereon (hydrogenpressure: 6.5 MPaG; liquid hourly space velocity (LHSV): 2.5 h⁻¹;temperature: 315° C.; desulfurization rate: 48%) to remove lightcontents therefrom, and then subjected to solvent dewaxing (methyl ethylketone: toluene=1:1; solvent ratio: 330%; cooled to −20° C.; yield: 84%)to obtain a high-viscosity base oil having a dynamic viscosity (40° C.)of 508.4 mm²/s, a pour point of −10° C. and a viscosity index of 101.

[0082] The extract thus obtained exhibited the content of PCA extract of2.7% by mass as measured by IP346 method, a percent C_(A) of 25.3%, adynamic viscosity (100° C.) of 65.26 mm²/s, an aniline point of 72° C.,a chromatographically-determined aromatic content of 84% by weight andMW of 785.

COMPARATIVE EXAMPLE 1

[0083] The atmospheric residue of Arabian light crude oil was distilledunder reduced pressure until the end point (gas chromatographicdistillation FBP) reached 600° C. The resulting residue was thendeasphalted with propane (solvent ratio: 700%; pressure: 3.3 MPaG;reaction column temperature: 72° C.) so that the carbon residue contentreached 1.3%. The deasphalted oil was then subjected to solventextraction with furfural as a solvent at a solvent ratio of 350% so thatthe yield of extract reached 30%.

[0084] The extract thus obtained exhibited the content of PCA extract of4.0% by mass as measured by IP346 method, a percent C_(A) of 28.6%, adynamic viscosity (100° C.) of 80.24 mm²/s, an aniline point of 63° C.,a chromatographically-determined aromatic content of 86% by weight andMW of 730.

COMPARATIVE EXAMPLE 2

[0085] The atmospheric residue of Arabian light crude oil was distilledunder reduced pressure until the end point (gas chromatographicdistillation FBP) reached 600° C. The resulting residue was thendeasphalted with propane (solvent ratio: 700%; pressure: 3.3 MPaG;reaction column temperature: 72° C.) so that the carbon residue contentreached 1.3%. The deasphalted oil was then subjected to solventextraction with furfural as a solvent at a solvent ratio of 280% so thatthe yield of extract reached 20%.

[0086] The extract thus obtained exhibited the content of PCA extract of5.3% by mass as measured by IP346 method, a percent CA of 33.5%, adynamic viscosity (100° C.) of 110.6 mm²/s, an aniline point of 51° C.,a chromatographically-determined aromatic content of 86% by weight andMW of 645.

COMPARATIVE EXAMPLE 3

[0087] The atmospheric residue of Arabian light crude oil was distilledunder reduced pressure until the end point (gas chromatographicdistillation FBP) reached 560° C. The resulting residue was thendeasphalted with propane (solvent ratio: 700%; pressure: 3.3 MPaG;reaction column temperature: 720° C.) so that the carbon residue contentreached 1.3%. The deasphalted oil was then subjected to solventextraction with furfural as a solvent at a solvent ratio of 280% so thatthe yield of extract reached 25%.

[0088] The extract thus obtained exhibited the content of PCA extract of9.9% by mass as measured by IP346 method, a percent C_(A) of 33.6%, adynamic viscosity (100° C.) of 58.33 mm²/s, an aniline point of 55° C.,a chromatographically-determined aromatic content of 86% by weight andMW of 601.

[0089] The conditions under which solvent refining was conducted inthese examples and comparative examples and the properties of thehigh-viscosity lubricating base oils obtained by solvent dewaxing of theresulting extracts and raffinates are set forth in Table 1 below. TABLE1 Comparative Comparative Comparative Example Example Example Example 11 2 3 Process Gas chromatographic 500 600 600 560 oil distillation FBPSolvent ratio 400 350 280 280 Yield (%) 42 30 20 25 Density (15° C.)g/cm³ 0.9716 0.9853 1.0094 0.9994 Dynamic viscosity 226.7 304.9 485.4213.5 (75° C.) mm²/g Dynamic viscosity 65.26 80.24 110.6 58.33 (100° C.)mm²/g Nitrogen content mass-% 0.11 0.14 0.15 0.16 Aniline point (° C.)72 63 51 55 PCA extract 2.7 4.0 5.3 9.9 mass-% Mw (weight-average 785730 645 601 molecular weight) Refractive index (nD20) 1.5432 1.55221.5671 1.5634 VGC 0.9005 0.9160 0.9535 0.9464 RI 1.0589 1.0611 1.06391.0652 % C_(A) 25.3 28.6 33.5 33.6 Aromatic content wt-% 84 86 86 86Glass transition point −45 −42 −42 −42 (° C.) Mutagenicity index (MI) <1<1 <1 ≧1 High- Density (15° C.) g/cm³ 0.8940 0.8977 0.9011 — viscosityDynamic viscosity 508.4 520.9 510.7 — base (40° C.) mm²/g oil Dynamicviscosity 34.12 33.91 33.00 — (100° C.) mm²/g Viscosity index 101 98 97— Sulfur content mass-% 0.50 0.61 0.75 — Aniline point ° C. 128 124 122— Pour point ° C. −10 −10 −10 —

[0090] As mentioned above, the production process of the invention makesit possible to obtain a process oil having a high safety and a highpenetrating power with respect to rubber polymer and a high-viscositybase oil at the same time and a reduced cost.

[0091] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

[0092] This application is based on Japanese patent application No.2000-117447, filed on Apr. 19, 2000, and incorporated herein byreference.

What is claimed is:
 1. A process for the production of an extract usefulas a process oil and a raffinate useful as a high-viscosity base oil bysolvent refining, which comprises carrying out reduced pressuredistillation under the condition that the end point of distillate asconverted to the value under atmospheric pressure is 580° C. or higheror the initial boiling point of the residue is 450° C. or higher ascalculated in terms of atmospheric pressure, deasphalating the resultingresidual oil under the condition that the carbon residue content in thedeasphalted oil reaches 1.6% or less, and subjecting the resultingdeasphalted oil to solvent refining under the condition that the yieldof extract is from 35% to 60%.
 2. The production process according toclaim 1 , wherein said extract useful as a process oil exhibits a 100°C. dynamic viscosity of from 50 to 100 mm²/s, a percent C_(A) of from15% to 35%, the content of PCA extract (IP346) of less than 3%, ananiline point of 90° C. or lower, a chromatographically-determinedaromatic content of from 60% to 95% by weight and Mw (weight-averagemolecular weight) of 650 or more.
 3. The production process according toclaim 1 , wherein the high-viscosity base oil having a 40° C. dynamicviscosity of from not lower than 400 mm²/s to not higher than 700 mm²/sobtained after the dewaxing of raffinate exhibits a pour point of nothigher than −5° C. and a viscosity index of not lower than
 95. 4. Aprocess oil having a 100° C. dynamic viscosity of from 50 to 100 mm²/s,a percent C_(A) of from 15% to 35%, the content of PCA extract (IP346)of less than 3%, an aniline point of 90° C. or lower, achromatographically-determined aromatic content of from 60% to 95% byweight and Mw (weight-average molecular weight) of 650 or more.
 5. Theprocess oil according to claim 4 , which has a mutagenicity index MI ofless than 1.